1. Field of the Invention
The present invention relates generally to network systems for transmitting power and data between device nodes. More particularly, the invention relates to a system media architecture incorporating modular connector devices coupled to a shared network conductor cable via insulation displacement members, permitting devices coupled at each connector node to draw both power and data from the cable.
2. Description of the Related Art
A variety of control systems are known and are presently used in industry for communicating control and feedback signals between remote controllers, sensors and actuators. In a typical application, a process, such as a manufacturing or assembly line, will include a number of sensors for providing information relating to the manufacturing process, such as speeds of conveyors, speeds of motors, temperatures, pressures, feed rates, fluid levels, logical states of switches, and so forth. The sensed information is transmitted from the sensors to one or more control units which contain logic devices for processing the signals. A number of actuators will also typically be included for executing particular process functions or controlling phases of the process. The actuators might include motor controllers, electric relays, solenoid coils, and so forth. In most modern applications, the control circuit will include one or more microprocessors and related solid state memory devices and other circuitry, appropriately programmed in accordance with the specific application. Based upon the sensed information and upon the program being executed by the controller, the controller will generate and transmit control or command signals to the actuators for carrying out the desired process. In many applications, a large number of sensed parameters and controlled actuators may be included to implement and control many facets of a manufacturing or other process.
While control and monitoring systems of the type described above may be "hard-wired", via power and signal conductors extending directly between sensors, actuators and a control circuit, an increasing number of systems are configured to communicate power and data over a networked system architecture. Networked systems may be divided into a first class in which data alone is transmitted between devices, and a second class in which both power and data signals are transmitted to networked devices. In network systems configured for transmitting only data between devices, large amounts of data, typically in the form of digitized or pulsed signals, may be transmitted between various devices over typical multi-strand ribbon cables. However, such cables are generally unsuitable for industrial power levels and distances. In many industrial applications networked devices must be powered by some source as well as communicate and receive data signals. In such cases, it is usually preferred to transmit both power and data via a single medium, such as a multi-conductor shielded cable. The cable may be of substantial length and constructed to provide reliable data transmission capabilities as well as power to a large number of device nodes. Connectors for each device node are coupled to the cable to enable the device to place and receive digitized data messages on the network as well as to receive power from the cable power conductors. An industrial network of this type is commercially available from the Allen-Bradley Company of Milwaukee, Wis. under the commercial designation DeviceNet.
While such networks provide excellent data transmission capabilities as well as enable devices to be powered through a single trunk cable, they are not without drawbacks. For example, the installation of the trunk cable at each device node may be a relatively time-consuming task, typically requiring each conductor to be separately wired to the node connector. Moreover, because the trunk cable is shielded and conductors are wound or twisted within the cable to reduce the effects of internal and external noise, alternative attachment techniques, such as insulation displacement technology, are generally not readily applicable to the trunk cable.
In another commercially available industrial network system, a cable including a pair of conductors is pierced by insulation displacing pins at each device node or tap. Power is modulated with digitized or pulsed data through the two cable conductors and must be separated from the power by specialized circuitry. The cable insulation is designed to heal itself upon removal of a node connector to prevent shorting and corrosion of the conductors, such as when the cable is wetted. The node connectors are configured to be placed at any position along the cable and to interface with particular styles of device cables. The device cables extend between the trunk cable connector and the end device. Where a different type or style of device or device cable is needed, the entire trunk cable connector is removed from the system and replaced.
While the foregoing structure offers the facility of insulation displacement technology for assembly of the node connectors to the trunk cable, it too has significant drawbacks. For example, each node and power supply typically requires specialized circuitry for modulating, demodulating and prevention corruption of data signals transmitted along the cable conductors. Also, because the cable is asymmetrical, connectors can be placed on one side of the cable only, reducing the flexibility of the system installation. Moreover, when a connector is removed from the trunk cable, the system relies upon the self-healing feature of the cable jacket to prevent shorting of the conductors. If the cable jacket fails to sufficiently heal itself, the reliability of the network, including both power and data transmission, is jeopardized. Furthermore, where a device cable style needs to be changed, the entire connector is typically replaced, reducing the flexibility of the media system and imposing further costs on the system user.
There is a need, therefore, for an improved network system for transmitting electrical power and data between devices such as sensors, actuators and controllers. In particular, there is a need for a reliable data transmission network architecture which does not require specialized signal processing circuitry at each node for removing data from power signals. There is also a need for network architecture in which node connectors can be placed on a trunk cable in a straightforward installation requiring little time, and in which the connectors can then be utilized in a universal fashion with interface modules adapted for various devices and device cable types.